Continuous wave (CW) radio frequency (rf) (100-300 MHz) EPR has been developed in many laboratories, for example for the detection and imaging of free radicals in small rats. Currently this technique presents some limitations under in vivo conditions, such as limited sensitivity (5-50 .mu.M) and slow acquisition rate typically 5 minutes, to obtain a two-dimensional (2-D) image.
The use of pulsed EPR techniques overcomes current limitations. However, in contrast to NMR, the development of pulsed EPR has been very slow. The main reason is that the electron spin relaxation times (T.sub.1 and T.sub.2) are orders of magnitude shorter (100 ns to 10 .mu.s) than the typical nuclear relaxation times (10 ms to 1 s). These shorter relaxation times place extreme demands on experimental and technical conditions such as pulse duration, pulse power, instrumental dead time, and digital electronics hardware. Only in the last decade or so have pulsed EPR instruments, operating at X-band (9 GHz), been proposed and only very recently become commercially available.
A pulsed EPR spectrometer specifically developed for imaging applications and operating at 300 MHz was proposed by Bourg et al., in J. Magnetic Resonance B, 102, 112 (1993). In the design of this instrument the classical NMR-type duplex configuration--a small coil transmitter/receiver with passive receiver protection--was employed. A very small solenoid coil (four-turn coil, 8 mm in diameter) was used. This ensured a very high efficiency factor, .beta., of the coil defined as: .beta.=(B.sub.1 /.sqroot.P), where B.sub.1 is the amplitude of the rf magnetic field and P is the power incident on the coil. The device allowed operation with a very small sample (.apprxeq.2 ml) and at low rf power levels and, consequently, a short instrumental dead time (T.sub.D) was obtained.
Unfortunately, the design followed by Bourg et al., is not suitable when large samples (10 to 100 ml) are used. In fact, the efficiency factor of the resonator decreases with the sample volume, requiring the use of high rf power. Because the dead time increases with the rf power, it can become very difficult, if not impossible, to detect the free induction decay (FID) signal of a paramagnetic species with short relaxation times. This represents a strong instrumental limitation for the development of pulsed rf EPR and reduces the potential biological applications of the technique. A similar problem has been experienced in solid nuclear magnetic resonance apparatus.
A number of problems are suffered by Magnetic Resonance Imagers and Spectrometers which degrade images and signals. In order to obtain adequate signals and good signal-to-noise ratio (SNR), it is necessary to have fast rf pulse application, a high rf power input capability and efficient resonators. In addition to this it is highly desirable to have fast data acquisition and electronic processing capability as well as a short instrumental dead time. The dead time of an instrument limits its ability to detect any events until it recovers. Accordingly when an instrument is in its dead time it is unable to detect anything of use.
The primary source of instrumental dead time is the ringing of the cavity or resonator, and the dead time of a magnetic resonance instrument (T.sub.D) can be expressed as: EQU T.sub.D =.tau..sub.D .multidot.1n (P*.sub.TX /P.sub.NOISE)
where P*.sub.TX is the transmitted power leaking to the receiver and P.sub.NOISE is the detector noise. .tau..sub.D is the resonator ringing time and is itself expressed as: EQU .tau..sub.D =1/.pi..DELTA.v
where .DELTA.v is the bandwidth of the resonator.
There are a number of ways in which dead-time can be reduced. For example, the bandwidth (.DELTA.v) of the resonator can be increased by reducing the quality factor (Q). However, this has the effect of reducing sensitivity and, because more transmitter power is required, more power can leak to the receiver. Overcoupling of the resonator tends to reduce the dead-time, but gives rise to harmonics and interference due to reflected waves. Such interference can damage amplifiers and there is a risk that more power will leak into the receiver coil. It is known that the ringing time of a resonator may be reduced by reversing the RF pulse (i.e. phase shifting it by 180.degree.) for a short time period at the end of the main pulse. This `quenching` pulse results in the microwave magnetic field B.sub.1 in the cavity tending not to zero but to a negative value in the rotating frame with the same time constant. During this process B.sub.1 crosses the zero value. If tie quenching pulse is switched off right at the moment of zero crossing there is no more energy left in the cavity to radiate. This technique therefore results in at least partial cancellation of the ringing of the resonator, but its effects are limited.
Alternatively, electronic equipment may be enhanced to reduce the dead-time, but this merely addresses the symptoms of prolonged dead-time, not the cause. Consequently there will always be a limit imposed on the technique.